The invention relates to electronic music generation and, more particularly, to a frequency following circuit for deriving a control voltage proportional to the fundamental frequency (pitch) of a periodic input signal of a complex waveform.
An electrical circuit which produces a control voltage which follows the fundamental frequency of an electrical input signal, generated, for example, by a musical sound wave, may be utilized to determine the frequencies of one or more appropriately scaled voltage controlled-oscillators. The oscillators may be connected to drive musical sound producers so that the frequencies of the oscillators retain a fixed musical interval relationship with respect to the input signal.
When using such a frequency following circuit for musical purposes, it is desirable that the frequency control voltage be derived as soon as possible upon input of the musical signal. Changes in the frequency of the musical signal should create corresponding changes in the control voltage with a minimum delay. If the delay is too long, the listener will receive a disturbing time lag between the change in pitch of the input signal and the change in pitch of the output of the voltage-controlled oscillators.
Also, it is desirable that the control voltage should not change in response to random changes in the input signal, that is, the control voltage should not respond to noise such as the breathy starting transient of a wind instrument tone.
The problem presented in deriving a control voltage proportional to the pitch of the musical tone of complex waveform, is the preparation of the wave form so that its fundamental frequency can be extracted. The prior art has employed a method wherein the input signal is passed through one or more low pass filters which attenuate the higher harmonics of the signal faster than the filters attenuate the fundamental frequency. This scheme has the disadvantage that it introduces a time delay in the output by virtue of the phase shift of the low pass filters. It also boosts line hum and other unwanted low frequency signals.